May be not exactly true - see this:
http://mg.pov.lt/maemo-irclog/%23mae...01-07T21:49:22

So, basically, it should work without problems, except for videos (too low bandwidth via USB) - that can be achieved by using N900 vfat as mass storage mode to raspberry pi host - and, it's unknown if it would work or not for OpenGLES applications (Neverball). Everything else should work fine for any sane resolution, and without slowdowns.

Possible bonus is sending sound via pulseaudio (or, as proposed by joerg_rw, usb audio card emulation, whatever that means ), so HDMI capabilities of Raspberry Pi would be 100% used.

Personally, I can't wait to grab one and start experimenting...

/Estel